Solar-Driven Sustainability: III-V Semiconductor for Green Energy Production Technologies

Long-term societal prosperity depends on addressing the world's energy and environmental problems, and photocatalysis has emerged as a viable remedy. Improving the efficiency of photocatalytic processes is fundamentally achieved by optimizing the effective utilization of solar energy and enhancing the efficient separation of photogenerated charges. It has been demonstrated that the fabrication of III-V semiconductor-based photocatalysts is effective in increasing solar light absorption, long-term stability, large-scale production and promoting charge transfer. This focused review explores on the current developments in III-V semiconductor materials for solar-powered photocatalytic systems. The review explores on various subjects, including the advancement of III-V semiconductors, photocatalytic mechanisms, and their uses in H2 conversion, CO2 reduction, environmental remediation, and photocatalytic oxidation and reduction reactions. In order to design heterostructures, the review delves into basic concepts including solar light absorption and effective charge separation. It also highlights significant advancements in green energy systems for water splitting, emphasizing the significance of establishing eco-friendly systems for CO2 reduction and hydrogen production. The main purpose is to produce hydrogen through sustainable and ecologically friendly energy conversion. The review intends to foster the development of greener and more sustainable energy source by encouraging researchers and developers to focus on practical applications and advancements in solar-powered photocatalysis.

Organometal Halide Perovskite-Based Photoelectrochemical Module Systems for Scalable Unassisted Solar Water Splitting

Despite achievements in the remarkable photoelectrochemical (PEC) performance of photoelectrodes based on organometal halide perovskites (OHPs), the scaling up of small-scale OHP-based PEC systems to large-scale systems remains a great challenge for their practical application in solar water splitting. Significant resistive losses and intrinsic defects are major obstacles to the scaling up of OHP-based PEC systems, leading to the PEC performance degradation of large-scale OHP photoelectrodes. Herein, a scalable design of the OHP-based PEC systems by modularization of the optimized OHP photoelectrodes exhibiting a high solar-to-hydrogen conversion efficiency of 10.4% is suggested. As a proof-of-concept, the OHP-based PEC module achieves an optimal PEC performance by avoiding major obstacles in the scaling up of the OHP photoelectrodes. The constructed OHP module is composed of a total of 16 OHP photoelectrodes, and a photocurrent of 11.52 mA is achieved under natural sunlight without external bias. The successful operation of unassisted solar water splitting using the OHP module without external bias can provide insights into the design of scalable OHP-based PEC systems for future practical application and commercialization.

A Perovskite Photovoltaic Mini-Module-CsPbBr3 Photoelectrochemical Cell Tandem Device for Solar-Driven Degradation of Organic Compounds

Recently, halide perovskites have been widely explored for high-efficiency photocatalysis or photoelectrochemical (PEC) cells. Here, in order to make an efficient photoanode electrode for the degradation of pollutants, concretely 2-mercaptobenzothiazole (MBT), nanoscale cesium lead bromide (CsPbBr3) perovskite was directly formed on the surface of mesoporous titanium dioxide (meso-TiO2) film using a two-step spin-coating process. This photoelectrode recorded a photocurrent of up to 3.02 ± 0.03 mA/cm2 under standard AM 1.5G (100 mW/cm2) illumination through an optimization process such as introducing a thin aluminum oxide (Al2O3) coating layer. Furthermore, to supply high voltage for efficient oxidation of MBT without an external bias, we developed a new photovoltaic/PEC tandem system using a methylammonium lead iodide (MAPbI3) based mini-module consisting of three solar cells interconnected in series and confirmed its successful operation. This approach looks very promising due to its applicability to various PEC reactions.

Tandem cells for unbiased photoelectrochemical water splitting

Hydrogen is an essential energy carrier which will address the challenges posed by the energy crisis and climate change. Photoelectrochemical water splitting (PEC) is an important method for producing solar-powered hydrogen. The PEC tandem configuration harnesses sunlight as the exclusive energy source to drive both the hydrogen (HER) and oxygen evolution reactions (OER), simultaneously. Therefore, PEC tandem cells have been developed and gained tremendous interest in recent decades. This review describes the current status of the development of tandem cells for unbiased photoelectrochemical water splitting. The basic principles and prerequisites for constructing PEC tandem cells are introduced first. We then review various single photoelectrodes for use in water reduction or oxidation, and highlight the current state-of-the-art discoveries. Second, a close look into recent developments of PEC tandem cells in water splitting is provided. Finally, a perspective on the key challenges and prospects for the development of tandem cells for unbiased PEC water splitting are given.

An integrated n-Si/BiVO4 photoelectrode with an interfacial bi-layer for unassisted solar water splitting

Integrated n-Si/BiVO4 is one of the most promising candidates for unbiased photoelectrochemical water splitting. However, a direct connection between n-Si and BiVO4 will not attain overall water splitting due to the small band offset as well as the interfacial defects at the n-Si/BiVO4 interface that severely impede carrier separation and transport, limiting the photovoltage generation. This paper describes the design and fabrication of an integrated n-Si/BiVO4 device with enhanced photovoltage extracted from the interfacial bi-layer for unassisted water splitting. An Al2O3/indium tin oxide (ITO) interfacial bi-layer was inserted at the n-Si/BiVO4 interface, which promotes the interfacial carrier transport by enlarging the band offset while healing interfacial defects. When coupled to a separate cathode for hydrogen evolution, spontaneous water splitting could be realized with this n-Si/Al2O3/ITO/BiVO4 tandem anode, with an average solar-to-hydrogen (STH) efficiency of 0.62% for over 1000 hours.

Inter-facet junction effects on particulate photoelectrodes

Particulate semiconductor photocatalysts are paramount for many solar energy conversion technologies. In anisotropically shaped photocatalyst particles, the different constituent facets may form inter-facet junctions at their adjoining edges, analogous to lateral two-dimensional (2D) heterojunctions or pseudo-2D junctions made of few-layer 2D materials. Using subfacet-level multimodal functional imaging, we uncover inter-facet junction effects on anisotropically shaped bismuth vanadate (BiVO₄) particles and identify the characteristics of near-edge transition zones on the particle surface, which underpin the whole-particle photoelectrochemistry. We further show that chemical doping modulates the widths of such near-edge surface transition zones, consequently altering particles' performance. Decoupled facet-size scaling laws further translate inter-facet junction effects into quantitative particle-size engineering principles, revealing surprising multiphasic size dependences of whole-particle photoelectrode performance. The imaging tools, the analytical framework and the inter-facet junction concept pave new avenues towards understanding, predicting and engineering (opto)electronic and photoelectrochemical properties of faceted semiconducting materials, with broad implications in energy science and semiconductor technology.

Semiconducting Polymers for Oxygen Evolution Reaction under Light Illumination

Sunlight-driven water splitting to produce hydrogen fuel has stimulated intensive scientific interest, as this technology has the potential to revolutionize fossil fuel-based energy systems in modern society. The oxygen evolution reaction (OER) determines the performance of overall water splitting owing to its sluggish kinetics with multielectron transfer processing. Polymeric photocatalysts have recently been developed for the OER, and substantial progress has been realized in this emerging research field. In this Review, the focus is on the photocatalytic technologies and materials of polymeric photocatalysts for the OER. Two practical systems, namely, particle suspension systems and film-based photoelectrochemical systems, form two main sections. The concept is reviewed in terms of thermodynamics and kinetics, and polymeric photocatalysts are discussed based on three key characteristics, namely, light absorption, charge separation and transfer, and surface oxidation reactions. A satisfactory OER performance by polymeric photocatalysts will eventually offer a platform to achieve overall water splitting and other advanced applications in a cost-effective, sustainable, and renewable manner using solar energy.

Elaborately Modified BiVO4 Photoanodes for Solar Water Splitting

Photoelectrochemical (PEC) cells for solar-energy conversion have received immense interest as a promising technology for renewable hydrogen production. Their similarity to natural photosynthesis, utilizing sunlight and water, has provoked intense research for over half a century. Among many potential photocatalysts, BiVO₄ , with a bandgap of 2.4-2.5 eV, has emerged as a highly promising photoanode material with a good chemical stability, environmental inertness, and low cost. Unfortunately, its charge transport properties are modest, at most a hole diffusion length (L_p ) of ≈70 nm. However, recent rapid developments in multiple modification strategies have elevated it to a position as the most promising metal oxide photoanode material. This review summarizes developments in BiVO₄ photoanodes in the past 10 years, in which time it has continuously broken its own performance records for PEC water oxidation. Effective modification techniques are discussed, including synthesis of nanostructures/nanopores, external/internal doping, heterojunction fabrication, surface passivation, and cocatalysts. Tandem systems for unassisted solar water splitting and PEC production of value-added chemicals are also discussed.

Toward practical solar hydrogen production - an artificial photosynthetic leaf-to-farm challenge

Solar water splitting is a promising approach to transform sunlight into renewable, sustainable and green hydrogen energy. There are three representative ways of transforming solar radiation into molecular hydrogen, which are the photocatalytic (PC), photoelectrochemical (PEC), and photovoltaic-electrolysis (PV-EC) routes. Having the future perspective of green hydrogen economy in mind, this review article discusses devices and systems for solar-to-hydrogen production including comparison of the above solar water splitting systems. The focus is placed on a critical assessment of the key components needed to scale up PEC water splitting systems such as materials efficiency, cost, elemental abundancy, stability, fuel separation, device operability, cell architecture, and techno-economic aspects of the systems. The review follows a stepwise approach and provides (i) a summary of the basic principles and photocatalytic materials employed for PEC water splitting, (ii) an extensive discussion of technologies, procedures, and system designs, and (iii) an introduction to international demonstration projects, and the development of benchmarked devices and large-scale prototype systems. The task of scaling up of laboratory overall water splitting devices to practical systems may be called "an artificial photosynthetic leaf-to-farm challenge".

Progress on ternary oxide-based photoanodes for use in photoelectrochemical cells for solar water splitting

Solar water splitting using photoelectrochemical cells (PECs) has emerged as one of the most promising routes to produce hydrogen as a clean and renewable fuel source. Among various semiconductors that have been considered as photoelectrodes for use in PECs, oxide-based photoanodes are particularly attractive because of their stability in aqueous media in addition to inexpensive and facile processing compared to other types of semiconductors. However, they typically suffer from poor charge carrier separation and transport. In the past few years, there has been tremendous progress in developing ternary oxide-based photoelectrodes, specifically, photoanodes. The use of ternary oxides provides more opportunities to tune the composition and electronic structure of the photoelectrode compared to binary oxides, thus providing more freedom to tune the photoelectrochemical properties. In this article, we outline the important characteristics to analyze when evaluating photoanodes and review the major recent progress made on the development of ternary oxide-based photoanodes. For each system, we highlight the favorable and unfavorable features and summarize the strategies utilized to address the challenges associated with each material. Finally, by combining our analyses of all the photoanodes surveyed in this review, we provide possible future research directions for each compound and an outlook for constructing more efficient oxide-based PECs. Overall, this review will provide a critical overview of current ternary oxide-based photoanodes and will serve as a platform for the design of future oxide-based PECs.

Simultaneous Photoelectrocatalytic Water Oxidation and Oxygen Reduction for Solar Electricity Production in Alkaline Solution

The photoelectrochemical (PEC) method offers an alternative approach to photovoltaic devices for solar electricity generation. The water oxidation reaction (WOR) on the anode and oxygen reduction reaction (ORR) on the cathode is an ideal design for energy transfer owing to their superiority in terms of cleanliness, eco-friendliness, and natural abundance. However, solar electricity production based on O₂ circulation by a fuel-free PEC cell is very challenging because it is extremely hard to extract electrons from water molecules owing to the uphill and sluggish WOR together with enormous overpotential for the cathodic ORR. Herein, a PEC cell based on the OH^- /O₂ redox pair is reported for efficient and sustainable solar electricity production by using two photoelectrodes of TiO₂ and polyterthiophene in alkaline electrolyte. This fuel-free PEC cell delivers an open-circuit voltage up to 0.90 V and a maximum power density of 222 μW cm^-2 with O₂ -saturated NaOH electrolyte under AM 1.5 G solar irradiation. A record solar-to-electricity conversion efficiency of 0.22 % is achieved in the case of tandem illumination of the two photoelectrodes. In addition, the dual photoelectrode remains robust in accelerated and day-night cycling operation under natural atmosphere for more than a week. This PEC cell is free of fuel, separating membranes, and cocatalyst, which may guide future designs for clean and simple devices for solar energy conversion.

Rational Design and Construction of Cocatalysts for Semiconductor-Based Photo-Electrochemical Oxygen Evolution: A Comprehensive Review

Photo-electrochemical (PEC) water splitting, as an essential and indispensable research branch of solar energy applications, has achieved increasing attention in the past decades. Between the two photoelectrodes, the photoanodes for PEC water oxidation are mostly studied for the facile selection of n-type semiconductors. Initially, the efficiency of the PEC process is rather limited, which mainly results from the existing drawbacks of photoanodes such as instability and serious charge-carrier recombination. To improve PEC performances, researchers gradually focus on exploring many strategies, among which engineering photoelectrodes with suitable cocatalysts is one of the most feasible and promising methods to lower reaction obstacles and boost PEC water splitting ability. Here, the basic principles, modules of the PEC system, evaluation parameters in PEC water oxidation reactions occurring on the surface of photoanodes, and the basic functions of cocatalysts on the promotion of PEC performance are demonstrated. Then, the key progress of cocatalyst design and construction applied to photoanodes for PEC oxygen evolution is emphatically introduced and the influences of different kinds of water oxidation cocatalysts are elucidated in detail. Finally, the outlook of highly active cocatalysts for the photosynthesis process is also included.

A microstructured p-Si photocathode outcompetes Pt as a counter electrode to hematite in photoelectrochemical water splitting

Herein, we communicate about an Earth-abundant semiconductor photocathode (p-Si/TiO2/NiOx) as an alternative for the rare and expensive Pt as a counter electrode for overall photoelectrochemical water splitting. The proposed photoelectrochemical (PEC) water-splitting device mimics the "Z"-scheme observed in natural photosynthesis by combining two photoelectrodes in a parallel-illumination mode. A nearly 60% increase in the photocurrent density (Jph) for pristine α-Fe2O3 and a 77% increase in the applied bias photocurrent efficiency (ABPE) were achieved by replacing the conventionally used Pt cathode with an efficient, cost effective p-Si/TiO2/NiOx photocathode under parallel illumination. The resulting photocurrent density of 1.26 mA cm-2 at 1.23VRHE represents a new record performance for hydrothermally grown pristine α-Fe2O3 nanorod photoanodes in combination with a photocathode, which opens the prospect for further improvement by doping α-Fe2O3 or by its decoration with co-catalysts. Electrochemical impedance spectroscopy measurements suggest that this significant performance increase is due to the enhancement of the space-charge field in α-Fe2O3.

Visible-Light-Responsive Photoanodes for Highly Active, Stable Water Oxidation

Solar energy is a natural and effectively permanent resource and so the conversion of solar radiation into chemical or electrical energy is an attractive, although challenging, prospect. Photo-electrochemical (PEC) water splitting is a key aspect of producing hydrogen from solar power. However, practical water oxidation over photoanodes (in combination with water reduction at a photocathode) in PEC cells is currently difficult to achieve because of the large overpotentials in the reaction kinetics and the inefficient photoactivity of the semiconductors. The development of semiconductors that allow high solar-to-hydrogen conversion efficiencies and the utilization of these materials in photoanodes will be a necessary aspect of achieving efficient, stable water oxidation. This Review discusses advances in water oxidation activity over photoanodes of n-type visible-light-responsive (oxy)nitrides and oxides.

Exploratory Study of Zn xPbO y Photoelectrodes for Unassisted Overall Solar Water Splitting

A complete photoelectrochemical (PEC) water splitting system requires a photocathode and a photoanode to host water oxidation and reduction reactions, respectively. It is thus important to search for efficient photoelectrodes capable of full water splitting. Herein, we report on an exploratory study of a new photoelectrode family of Zn _xPbO _y-ZnPbO₃ and Zn₂PbO₄-similarly synthesized by a simple and economical method and shown to be a promising photocathode (p-type semiconductor) and photoanode (n-type semiconductor), respectively. From PEC measurements, the bare ZnPbO₃ photocathode achieved a photocurrent density of -0.94 mA/cm² at 0 V versus reversible hydrogen electrode (RHE), whereas the pristine Zn₂PbO₄ photoanode delivered a photocurrent density of 0.51 mA/cm² at 1.23 V versus RHE. By depositing suitable cocatalysts onto the photoelectrodes established above, we also demonstrated unassisted overall PEC water splitting, a rare case, if any, wherein a single material system is compositionally engineered for either of the photoelectrodes.

Nano-designed semiconductors for electro- and photoelectro-catalytic conversion of carbon dioxide

This review describes a systematic overview on rational design of semiconductor catalysts for electro- and photoelectro-chemical CO₂ conversion.

A porphyrin cobalt(ii) complex linked to a TiO2/BiVO4 nanocomposite: alcohol oxidation using nanohybrid materials as a photocatalyst via a mechanism approach

A novel visible light-sensitized nanohybrid, Co–TCPP@TiO₂/BiVO₄, was used as a highly efficient, stable, reusable, and robust photocatalyst using O₂ towards aldehyde production.

Recent Advances in Photoelectrochemical Applications of Silicon Materials for Solar-to-Chemicals Conversion

Photoelectrochemical (PEC) technology for the conversion of solar energy into chemicals requires cost-effective photoelectrodes to efficiently and stably drive anodic and/or cathodic half-reactions to complete the overall reactions for storing solar energy in chemical bonds. The shared properties among semiconducting photoelectrodes and photovoltaic (PV) materials are light absorption, charge separation, and charge transfer. Earth-abundant silicon materials have been widely applied in the PV industry, and have demonstrated their efficiency as alternative photoabsorbers for photoelectrodes. Many efforts have been made to fabricate silicon photoelectrodes with enhanced performance, and significant progress has been achieved in recent years. Herein, recent developments in crystalline and thin-film silicon-based photoelectrodes (including amorphous, microcrystalline, and nanocrystalline silicon) immersed in aqueous solution for PEC hydrogen production from water splitting are summarized, as well as applications in PEC CO₂ reduction and PEC regeneration of discharged species in redox flow batteries. Silicon is an ideal material for the cost-effective production of solar chemicals through PEC methods.

Microbial Photoelectrosynthesis for Self-Sustaining Hydrogen Generation

Current artificial photosynthesis (APS) systems are promising for the storage of solar energy via transportable and storable fuels, but the anodic half-reaction of water oxidation is an energy intensive process which in many cases poorly couples with the cathodic half-reaction. Here we demonstrate a self-sustaining microbial photoelectrosynthesis (MPES) system that pairs microbial electrochemical oxidation with photoelectrochemical water reduction for energy efficient H₂ generation. MPES reduces the overall energy requirements thereby greatly expanding the range of semiconductors that can be utilized in APS. Due to the recovery of chemical energy from waste organics by the mild microbial process and utilization of cost-effective and stable catalyst/electrode materials, our MPES system produced a stable current of 0.4 mA/cm² for 24 h without any external bias and ∼10 mA/cm² with a modest bias under one sun illumination. This system also showed other merits, such as creating benefits of wastewater treatment and facile preparation and scalability.

Reverse Electrodialysis-Assisted Solar Water Splitting

Photoelectrochemical (PEC) water splitting provides an attractive route for large-scale solar energy storage, but issues surrounding the efficiency and the stability of photoelectrode materials impose serious restrictions on its advancement. In order to relax one of the photoelectrode criteria, the band gap, a promising strategy involves complementing the conventional PEC setup with additional power sources. Here we introduce a new concept: solar water splitting combined with reverse electrodialysis (RED). RED is a membrane-based power generation technology that produces an electrochemical potential difference from a salinity gradient. In this study, the RED stack serves not only as a separator, but also as an additional tunable power source to compensate for the limited voltage produced by the photoelectrode. A hybrid system, composed of a single-junction p-Si and a RED stack, successfully enables solar water splitting without the need for an external bias. This system provides flexibility in photoelectrode material selection.

TiO2-BiVO4 Heterostructure to Enhance Photoelectrochemical Efficiency for Sensitive Aptasensing

This work designed a nanocomposite to enhance the photoelectrochemical (PEC) efficiency by depositing BiVO₄ nanoparticles on TiO₂ nanospheres with a solvothermal method. The TiO₂-BiVO₄ heterostructure was characterized with various spectroscopic and microscopic techniques and was employed as a nanostructured support to cross-link DNA aptamer for constructing a visible-light driven PEC aptasensor. The TiO₂ nanospheres provided a biocompatible microenvironment, and the high surface area of the heterostructure enhanced the loading of aptamer molecules. The small energy gap of BiVO₄ improved the PEC property of the nanocomposite compared with the pure TiO₂ under visible-light irradiation. The advantages of the nanocomposite along with the high loading of recognition molecules greatly improved the sensitivity of the aptasensor. Using 17β-estradiol as an analyst model, the proposed PEC biosensor showed excellent analytical performance with high sensitivity, low detection limit of 0.022 pM, and high selectivity in a detectable concentration range of 0.1-250 pM, indicating the promising application of the designed TiO₂-BiVO₄ heterostructure in PEC biosensing.

Sunlight-Dependent Hydrogen Production by Photosensitizer/Hydrogenase Systems

We report a sustainable in vitro system for enzyme-based photohydrogen production. The [FeFe]-hydrogenase HydA1 from Chlamydomonas reinhardtii was tested for photohydrogen production as a proton-reducing catalyst in combination with eight different photosensitizers. Using the organic dye 5-carboxyeosin as a photosensitizer and plant-type ferredoxin PetF as an electron mediator, HydA1 achieves the highest light-driven turnover number (TON_HydA1 ) yet reported for an enzyme-based in vitro system (2.9×10⁶  mol(H₂ ) mol(cat)^-1 ) and a maximum turnover frequency (TOF_HydA1 ) of 550 mol(H₂ ) mol(HydA1)^-1  s^-1 . The system is fueled very effectively by ambient daylight and can be further simplified by using 5-carboxyeosin and HydA1 as a two-component photosensitizer/biocatalyst system without an additional redox mediator.

Design and Fabrication of a Dual-Photoelectrode Fuel Cell towards Cost-Effective Electricity Production from Biomass

A photo fuel cell (PFC) offers an attractive way to simultaneously convert solar and biomass energy into electricity. Photocatalytic biomass oxidation on a semiconductor photoanode combined with dark electrochemical reduction of oxygen molecules on a metal cathode (usually Pt) in separated compartments is the common configuration for a PFC. Herein, we report a membrane-free PFC based on a dual electrode, including a W-doped BiVO₄ photoanode and polyterthiophene photocathode for solar-stimulated biomass-to-electricity conversion. Air- and water-soluble biomass derivatives can be directly used as reagents. The optimal device yields an open-circuit voltage (V_OC ) of 0.62 V, a short-circuit current density (J_SC ) of 775 μA cm^-2 , and a maximum power density (P_max ) of 82 μW cm^-2 with glucose as the feedstock under tandem illumination, which outperforms dual-photoelectrode PFCs previously reported. Neither costly separating membranes nor Pt-based catalysts are required in the proposed PFC architecture. Our work may inspire rational device designs for cost-effective electricity generation from renewable resources.

Charge Transport in Two-Photon Semiconducting Structures for Solar Fuels

Semiconducting heterostructures are emerging as promising light absorbers and offer effective electron-hole separation to drive solar chemistry. This technology relies on semiconductor composites or photoelectrodes that work in the presence of a redox mediator and that create cascade junctions to promote surface catalytic reactions. Rational tuning of their structures and compositions is crucial to fully exploit their functionality. In this review, we describe the possibilities of applying the two-photon concept to the field of solar fuels. A wide range of strategies including the indirect combination of two semiconductors by a redox couple, direct coupling of two semiconductors, multicomponent structures with a conductive mediator, related photoelectrodes, as well as two-photon cells are discussed for light energy harvesting and charge transport. Examples of charge extraction models from the literature are summarized to understand the mechanism of interfacial carrier dynamics and to rationalize experimental observations. We focus on a working principle of the constituent components and linking the photosynthetic activity with the proposed models. This work gives a new perspective on artificial photosynthesis by taking simultaneous advantages of photon absorption and charge transfer, outlining an encouraging roadmap towards solar fuels.

Ultrafast fabrication of highly active BiVO4 photoanodes by hybrid microwave annealing for unbiased solar water splitting

Hybrid microwave annealing (HMA) with a silicon susceptor in a household microwave oven produces BiVO₄-based photoanodes of much improved performance in photoelectrochemical water oxidation in only 6 min relative to conventional thermal annealing in a traditional muffle furnace (FA) that needs a much longer time, 300 min. This technique can apply equally effectively to bare as well as modified BiVO₄ by Mo-doping, heterojunction formation with WO₃, and an oxygen evolution co-catalyst. Relative to FA, HMA forms BiVO₄ films of smaller feature sizes, higher porosity, and increased three dimensional roughness, which decrease the diffusion distance of holes to the surface and thereby increase mainly the bulk charge separation efficiency (η_bulk) of the photoanodes. Thus, the HMA-treated BiVO₄/WO₃ film achieves the state-of-the art η_bulk of ∼90% for water oxidation. Combination of a photoanode of NiOOH/FeOOH/BiVO₄/WO₃ (HMA, 6 min) with a 2p c-Si solar cell allows a solar to hydrogen conversion efficiency of ∼5.0% in unbiased overall water splitting, which is also comparable to the state-of-the-art for a similar material combination.

Hierarchically branched Fe2O3@TiO2 nanorod arrays for photoelectrochemical water splitting: facile synthesis and enhanced photoelectrochemical performance

Highly photoactive and durable photoanode materials are the key to photoelectrochemical water splitting. In this paper, hierarchically branched Fe2O3@TiO2 nanorod arrays (denoted as Fe2O3@TiO2 BNRs) composed of a long Fe2O3 trunk and numerous short TiO2 nanorod branches were fabricated and used as photoanodes for water splitting. Significant improvement of photoelectrochemical water splitting performance was observed based on Fe2O3@TiO2 BNRs. The photocurrent density of Fe2O3@TiO2 BNRs reaches up to 1.3 mA cm(-2) at 1.23 V versus RHE, which is 10 times higher than that of pristine Fe2O3 nanorod arrays under the same conditions. Furthermore, an obvious cathodic shift in the onset potential of photocurrent was observed in the Fe2O3@TiO2 BNRs. More significantly, the Fe2O3@TiO2 BNRs are quite stable even after 3600 s continuous illumination, and the photocurrent density shows almost no decay. Finally, a tentative mechanism was proposed to explain the superior performance of Fe2O3@TiO2 BNRs for PEC water splitting and discussed in detail on the basis of our experimental results.

Improving Stability and Photoelectrochemical Performance of BiVO4 Photoanodes in Basic Media by Adding a ZnFe2O4 Layer

BiVO4 photoanodes have mainly been investigated under neutral conditions because BiVO4 gradually dissolves under extreme pH conditions. In this study, the possibility of utilizing ZnFe2O4 as a protection layer to stabilize BiVO4 in a 0.1 M KOH solution was investigated. A 10-15 nm thick ZnFe2O4 layer was conformally placed on a nanoporous BiVO4 electrode by photodepositing a FeOOH layer, followed by drop casting a zinc nitrate solution and annealing. The resulting BiVO4/ZnFe2O4 electrode generated a photocurrent density of >2 mA/cm(2) at 1.23 V versus RHE with a significantly improved stability compared with the pristine BiVO4 electrode. The incident and absorbed photon-to-current conversion efficiencies along with absorption spectra suggested that the ZnFe2O4 protection layer also contributes to photocurrent generation by increasing photon absorption and electron-hole separation. These results suggest that further investigation of protection and catalyst layers can enable more stable and efficient operation of BiVO4-based photoanodes in basic media.

Overall Photoelectrochemical Water Splitting using Tandem Cell under Simulated Sunlight

A stand-alone photoelectrochemical (PEC) water-splitting system driven only by sunlight was demonstrated with a tandem-scheme of Pt/CdS/CuGa3 Se5 /(Ag,Cu)GaSe2 photocathode and NiOOH/FeOOH/Mo:BiVO4 photoanode in a neutral phosphate buffer solution as an electrolyte. The as-prepared semi-transparent Mo:BiVO4 layer allows sunlight to pass through the top photoanode and reach the bottom photocathode. Consequently, the tandem cell showed stoichiometric hydrogen and oxygen evolution with a solar-to-hydrogen (STH) conversion efficiency of 0.67 % over 2 h without degradation. The stability and STH efficiency are the highest among similar configuration of PEC tandem cells.

Integration of TiO2 photoanode and perovskite solar cell for overall solar-driven water splitting

Fully integrated device based on TiO₂ photoanode and perovskite solar cell for overall solar-driven water splitting.

Photoelectrochemical cell for unassisted overall solar water splitting using a BiVO4photoanode and Si nanoarray photocathode

Overall solar water splitting without external bias was demonstrated in this study using a photoelectrochemical tandem device of a BiVO₄photoanode and Si nanoarray photocathode.

Wireless Solar Water Splitting Device with Robust Cobalt-Catalyzed, Dual-Doped BiVO4 Photoanode and Perovskite Solar Cell in Tandem: A Dual Absorber Artificial Leaf

A stand-alone, wireless solar water splitting device without external energy supply has been realized by combining in tandem a CH3NH3PbI3 perovskite single junction solar cell with a cobalt carbonate (Co-Ci)-catalyzed, extrinsic/intrinsic dual-doped BiVO4 (hydrogen-treated and 3 at% Mo-doped). The photoanode recorded one of the highest photoelectrochemical water oxidation activity (4.8 mA/cm(2) at 1.23 VRHE) under simulated 1 sun illumination. The oxygen evolution Co-Ci co-catalyst showed similar performance to best known cobalt phosphate (Co-Pi) (5.0 mA/cm(2) at 1.23 VRHE) on the same dual-doped BiVO4 photoanode, but with significantly better stability. A tandem artificial-leaf-type device produced stoichiometric hydrogen and oxygen with an average solar-to-hydrogen efficiency of 4.3% (wired), 3.0% (wireless) under simulated 1 sun illumination. Hence, our device based on a D4 tandem photoelectrochemical cell represents a meaningful advancement in performance and cost over the device based on a triple-junction solar cell-electrocatalyst combination.

Conversion of Biomass Derivatives to Electricity in Photo Fuel Cells using Undoped and Tungsten-doped Bismuth Vanadate Photoanodes

The photo fuel cell (PFC) is a promising technology for simultaneously converting solar energy and bioenergy into electricity. Here, we present a miniature air-breathing PFC that uses either BiVO4 or W-doped BiVO4 as the photoanode and a Pt/C catalyst as the air-breathing cathode. The PFC exhibited excellent performance under solar illumination and when fed with several types of biomaterial. We found the PFC performance could be significantly enhanced using W-doping into the BiVO4 photoanode. With glucose as the fuel and simulated sunlight (AM 1.5 G) as the light source, the open-circuit voltage increased from 0.74 to 0.92 V, the short-circuit current density rose from 0.46 to 1.62 mA cm(-2) , and the maximum power density was boosted from 0.05 to 0.38 mW cm(-2) , compared to a PFC using undoped BiVO4 as the anode.

A Metal-Nitride Nanowire Dual-Photoelectrode Device for Unassisted Solar-to-Hydrogen Conversion under Parallel Illumination

A dual-photoelectrode device, consisting of a photoanode and photocathode with complementary energy bandgaps, has long been perceived as an ideal scheme for achieving high efficiency, unassisted solar-driven water splitting. Previously reported 2-photon tandem devices, however, generally exhibit an extremely low efficiency (<0.1%), which has been largely limited by the incompatibility between the two photoelectrode materials. Here we show that the use of metal-nitride nanowire photoelectrodes, together with the scheme of parallel illumination by splitting the solar spectrum spatially and spectrally, can break the efficiency bottleneck of conventional 2-photon tandem devices. We have first investigated a dual-photoelectrode device consisting of a GaN nanowire photoanode and an InGaN nanowire photocathode, which exhibited an open circuit potential of 1.3 V and nearly 20-fold enhancement in the power conversion efficiency under visible light illumination (400-600 nm), compared to the individual photoelectrodes in 1 mol/L HBr. We have further demonstrated a dual-photoelectrode device consisting of parallel-connected metal-nitride nanowire photoanodes and a Si/InGaN nanowire photocathode, which can perform unassisted, direct solar-to-hydrogen conversion. A power conversion efficiency of 2% was measured under AM1.5G 1 sun illumination.

Photoelectrochemical reaction for the efficient production of hydrogen and high-value-added oxidation reagents

A porous and thick photoelectrode of WO3 in the monoclinic phase was prepared to realize the recovery of H2 and high-value-added oxidation reagents with efficient solar energy conversion. The WO3 photoelectrode enabled the efficient production and accumulation of O2 , S2 O8 (2-) , Ce(4+) , and IO4 (-) as oxidation products. Most notably, S2 O8 (2-) , which possesses the highest oxidizability among all the peroxides, was generated with high applied bias photon-to-current efficiency (2.2 %) and faraday efficiency (≈100 %) upon irradiation from the back side of the photoelectrode. The design of a tandem photoelectrode system combining a dye-sensitized solar cell (DSSC) was also challenged for the realization of this photoelectrode system without external bias. A high solar energy conversion efficiency (5.2 %) was achieved in the tandem system comprising the WO3 photoelectrode connected to two DSSCs with a near-IR-utilizing dye in series for the production of H2 and S2 O8 (2-) .

High efficiency solar-to-hydrogen conversion on a monolithically integrated InGaN/GaN/Si adaptive tunnel junction photocathode

H2 generation under sunlight offers great potential for a sustainable fuel production system. To achieve high efficiency solar-to-hydrogen conversion, multijunction photoelectrodes have been commonly employed to absorb a large portion of the solar spectrum and to provide energetic charge carriers for water splitting. However, the design and performance of such tandem devices has been fundamentally limited by the current matching between various absorbing layers. Here, by exploiting the lateral carrier extraction scheme of one-dimensional nanowire structures, we have demonstrated that a dual absorber photocathode, consisting of p-InGaN/tunnel junction/n-GaN nanowire arrays and a Si solar cell wafer, can operate efficiently without the strict current matching requirement. The monolithically integrated photocathode exhibits an applied bias photon-to-current efficiency of 8.7% at a potential of 0.33 V versus normal hydrogen electrode and nearly unity Faradaic efficiency for H2 generation. Such an adaptive multijunction architecture can surpass the design and performance restrictions of conventional tandem photoelectrodes.

Effect of the Si/TiO2/BiVO4 heterojunction on the onset potential of photocurrents for solar water oxidation

BiVO4 has been formed into heterojunctions with other metal oxide semiconductors to increase the efficiency for solar water oxidation. Here, we suggest that heterojunction photoanodes of Si and BiVO4 can also increase the efficiency of charge separation and reduce the onset potential of the photocurrent by utilizing the high conduction band edge potential of Si in a dual-absorber system. We found that a thin TiO2 interlayer is required in this structure to realize the suggested photocurrent density enhancement and shifts in onset potential. Si/TiO2/BiVO4 photoanodes showed 1.0 mA/cm(2) at 1.23 V versus the reversible hydrogen electrode (RHE) with 0.11 V (vs RHE) of onset potential, which were a 3.3-fold photocurrent density enhancement and a negative shift in onset potential of 300 mV compared to the performance of FTO/BiVO4 photoanodes.